There is significant interest in improving display technology for use in a variety of consumer products. For example, HDTV technology uses high resolution displays. Large-format flat panel displays also are of increasing interest. Other examples of products which use display technologies include laptop computers and cell phones.
Display technology can be categorized into several types of systems. For example, there are direct view systems in which an image is viewed at a viewing surface proximal to the location where the image is produced. Examples of direct view systems include cathode ray tube displays, plasma displays, and liquid crystal displays (LCDs). There are also projection systems, in which an image is viewed by reflection and/or transmission on a surface remotely situated from where the image is produced. Examples of projection systems include rear projection televisions, front projectors, and cinema applications.
One example of a direct view system is a backlit LCD system. Direct view systems that utilize LCDs typically use a cold cathode fluorescent lamp (CCFL) as a light source to backlight the LCD. Monochrome LCD displays also may use electroluminescent lamps and/or organic LEDs (OLEDs) as backlights. In order to provide good illumination uniformity, the CCFL typically extends over a large portion of the area of the LCD panel, making it a large, expensive and fragile component within the display. Color filters which also extend across the full area of the screen typically are used to obtain red, green, and blue (RGB) colors from the CCFL, which is a white light source. The LCD panel then controls the transmission of the CCFL light through the display to convey an image to the viewer.
A conventional LCD panel is arranged as a two-dimensional matrix of pixels. In a backlit LCD system, a liquid crystal material controls the light transmission through each pixel of the LCD panel, typically in an analog fashion, from near zero to a maximum value. The transmissivity of the liquid crystal is changed by rotating the polarization state of light as it passes between two polarizers. The degree of polarization rotation induced by the liquid crystal material determines the amount of transmission and thus the brightness of a given pixel, and is controlled by electrically addressing each pixel. A direct view LCD system can include millions of individual pixels. For example, the XGA video format has a display resolution of 1,024×768 pixels, or 786,432 total pixels. There are many other available display formats, including several with higher resolutions.
Conventional polychromatic backlit LCD systems include pixels which typically are divided into 3 subpixels. Each subpixel has a color filter which allows transmission of a single color, which is typically red, green, or blue. The bandwidth of the filter transmission provides a trade-off between improved color saturation and definition for a narrower bandwidth, and a brighter display for a wider bandwidth. For example, a narrower filter bandwidth tends to provide richer, more saturated colors and a wider color gamut. However, as the filter bandwidth is decreased, the amount of light generated by the CCFL within that bandwidth decreases and therefore the display brightness decreases for a given CCFL power. A filter bandwidth of tens of nanometers allows reasonable transmission when using a broad spectrum illumination source such as a CCFL.
Red, green, and blue filters in polychromatic backlit LCD systems are typically arranged in alternate bands of vertical stripes which cover the entire image area. As an illustrative example, image formation may be achieved using scanning techniques similar to those used in CRT systems. For example, an image may be formed by adjusting transmission of pixels in a given row, such as the top row. The transmission of each subpixel is individually adjusted. The transmission of pixels in other rows are then adjusted according to a sequence. For example, the transmission of pixels in an adjacent row may be sequentially adjusted. Alternatively, a row could be skipped and returned to later in a subsequent scan using an interleaving pattern. In general, the procedure of adjusting the transmission in pixels of particular rows is continued until an entire image is scanned.
Conventional backlit LCD systems have several drawbacks. One drawback is that color gamut and color saturation are generally not sufficiently high, due to the trade-off between color filter bandwidth and display brightness. Also, color gamut can change with time as the CCFL or other lamp ages and the lamp output intensity spectrum changes. For example, a CCFL lamp has a limited lifetime before the overall output power decreases sufficiently to reduce the display brightness below acceptable levels. Further, the optical efficiency of such LCD systems is relatively low because most of the light is produced by the lamp is not transmitted by the color filters and/or polarization filters, and is therefore not used for displaying images.
There is increasing interest is using light emitting diodes (LEDs) as a light source for backlit LCD systems. LEDs can be up to 15% efficient at converting electricity into light. An array of LEDs can include diodes having red, green and blue emission wavelengths. Each LED typically has a center wavelength that varies from device to device (and from lot-to-lot) with a typical spectral width of about 20 to 30 nanometers. An LED can emit light over a comparatively large range of angles, often in a nearly hemispheric pattern. However, the combination of 3-color LED arrays and the color filters used in LCD panels tends to exhibit a limited color saturation and color gamut because the LED output is spectrally broad and therefore has difficulty recreating precise saturated colors. The combination of LED arrays and the color filters used in LCD panels provides a significant improvement in brightness and efficiency when compared to the use of a CCFL light source because less light energy is filtered out and thus unused in the color filters. However, as described above, the color reproduction and display quality may still not be sufficiently high.
Therefore, a need exists for methods and apparatus which can provide improved illumination of LCD panels which address the shortcomings of conventional LCD systems described above.